1. Field of the Invention
The present invention relates to a sheet conveyance device for conveying a sheet and an image forming apparatus provided with the sheet conveyance device.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine in which an electrophotographic system is used, a sheet such as recording paper in which an image is recorded is conveyed along a guide by rotation of a roller pair while nipped between the roller pair. The roller pairs which convey the sheet are arranged at an interval shorter than a length of the sheet in a conveyance direction, and the roller pair is rotated by a driving device such as a motor. A sheet conveyance speed of the sheet conveyed by the roller rotated by the motor controlled if necessary in such cases as the time recording is performed in the sheet, the time the sheet is fed from a sheet storage portion, and the time the sheet in which the recording is already performed is discharged. The sheet conveyance speed of the roller is increased, decreased or the roller is stopped in order to correct an attitude (such as skew and shift) of the sheet during the conveyance or to bring intervals between the sheets close to a predetermined setting value during the continuous conveyance.
The sheet conveyance control is performed based on detection information from a sheet detection sensor provided on a conveyance path. The sheet detection sensor detects a position of the sheet conveyed along the guide constituting the conveyance path. Examples of the sheet detection sensor include a flag sensor type and a photo sensor type. The sheet detection sensor provided on the conveyance path determines the position of the sheet by detecting passage of a leading end or a rear end of the sheet. When the sheet cannot be detected at predetermined conveyance timing, it is determined that conveyance failure such as jamming is generated (for example, see Japanese Patent Application Laid-Open No. 2002-347976)
As described above, the sheet detection sensor used in the sheet conveyance control is formed by the flag sensor type or the photo sensor type. Therefore from the standpoint of structure, it is difficult that the many sensors are continuously arranged in the conveyance path between the rollers. That is, in the flag sensor or the photo sensor, only one or two sensors are provided in the conveyance path between the rollers while separated away from each other due to the structure of the sensor, and the sheet is intermittently detected by the sensor. Particularly, in the case of downsizing the image forming apparatus, because the conveyance path is shortened or curved, sometimes the sensor cannot be provided depending on the conveyance path, and the arrangement of the sensor is further restricted. Thus, in the conventional configuration, because the detection point of the sensor is restricted, a status of the sheet cannot be traced except for the detection point, and it is difficult to finely perform the sheet conveyance control.
For example, it is assumed that, in order to bring an interval between the two continuous sheets close to a setting value, the sheet conveyance speed of the roller is increased or decreased based on information from the sensor located immediately after the roller for conveying the subsequent sheet. In this case, because the sheet detection sensors can intermittently be provided as described above, it cannot be determined whether or not the sheet interval is brought close to the setting value until the sheet passing by the sensor reaches the next sensor. Therefore, it is difficult to finely perform the sheet conveyance control.
In the conventional sheet conveyance control, it is necessary to perform the sheet conveyance control in consideration of the following fluctuations. Examples of a main generation factor of the fluctuation include conveyance delay caused by slip between the roller and the sheet, delay of a control signal from a controller to a motor or a clutch, a rising of falling shift of the motor, a shift of operation response, and shift of a stop position caused by an inertia force in stopping the conveyance. The fluctuations are indeterminately generated depending on the status of the conveyed sheet, environmental variations such as temperature and humidity, and simultaneously processed control contents Therefore, it is necessary that the sheet interval be set while a margin is ensured to permit the fluctuations, which results in a problem of decreasing productivity by the margin.